Locomotor adaptation to gradually and suddenly introduced force fields

People adapt differently to a dynamic or kinematic transformation introduced gradually than to the same transformation introduced suddenly. A gradual introduction, which prevents subjects from experiencing large movement errors, results in more complete adaptation and better short-term and long-term retention. These findings were all obtained in studies investigating arm movements. The aim of this study was to determine the effect of a gradually and suddenly introduced dynamic perturbation on locomotor adaptation and on recalling the adaptation when subsequently being re-exposed to the same perturbation. Sixteen participants were randomly assigned to two groups. Participants walked on a treadmill in the robotic gait trainer LOPES. This robotic apparatus was used to apply a downward force at the left ankle that was proportional to the forward velocity of the ankle during swing. This force field aimed to decrease the foot clearance during swing. The experiment consisted of 5 subsequent blocks, baseline (40 strides), first exposure (80 strides), first washout (120 strides), re-exposure (80 strides) and second washout (40 strides). For the gradual group the viscous force field was gradually introduced in the first exposure, whereas for the sudden group the force field was introduced suddenly. In the re-exposure block the force field was suddenly introduced for both groups. We derived the amount of adaptation from the foot clearance during mid-swing. A gradual introduction of the force field did not result in more complete adaptation, nor did it result in a slower decay of the adaptation during the washout periods. Strikingly, during re-exposure the gradual group showed hardly any adaptation whereas the sudden group showed an adaptation rate that was larger than during the first exposure (evidence for savings). In conclusion, a gradual introduction of the perturbation does not seem to have the same beneficial effects on locomotor adaptation than it has on adaptation in reaching. The observed difference in adaptation rates between the groups during re-exposure are in line with recent findings that time constants of adaptation are dependent on the statistics of the adaptation experience (Huang & Shadmehr, 2009).